In my prior application, Ser. No. 736,603 entitled "Canonical Realization of Single Mode Waveguide Bandpass Filter", now abandoned, I disclosed a folded structure for a single mode waveguide bandpass filter which generates the canonical set of couplings i, i+1 for i=1 to (n-1) and realizes the general class of coupled cavity transfer frunctions (e.g., elliptic). Such a filter is illustrated in FIG. 1(a) herein. The filter structure includes n=2 m cavities each resonant in the fundamental TE.sub.101 mode with input and output ports 12 and 14, respectively, provided at one end of the filter. Positive couplings are realized by magnetic coupling through slots 13 and negative coupling is achievd by circular holes 15. Note the canonical set of couplings i, i+1 for i=1 to (n-1) and the extra couplings 1 to n, 2 to n-1, 3 to n-2, etc. This coupling set represents the minimum possible number of couplings to achieve the general class of coupled cavity transfer functions.
In my prior application Ser. No. 616,479, now U.S. Pat. No. 3,969,692, I disclosed a circular TE.sub.011 high Q mode multiple coupled cavity waveguide bandpass filter capable of realizing the general class of multiple coupled cavity filter transfer functions. Such a filter structure, shown in FIGS. 1(b) and 1(bb), includes a plurality of circular cavities resonant in the TE.sub.011 mode with the input and output ports 12 and 14, respectively, at one end of the filter. This structure illustrates how negative couplings can be achieved in the high Q circular TE.sub.011 mode by an end cavity overlay of one-half a diameter. It is important to note that this structure, which satisfies the canonical coupling geometry, allows the series couplings i, i+1 for i=1 to (n-1) and the extra couplings 1 to n, 2 to n-1, 3 to n-2, etc. By offsetting the fifth cavity by a half diameter with respect to the fourth cavity, the sign of the numeral 3-6 coupling is opposite the sign of the numeral 4-5 coupling and the general class of filter functions can thus be achieved.
In my prior application Ser. No. 754,804 entitled "Canonical Dual Mode Filter", and now U.S. Pat. No. 4,060,779. I disclosed a filter which is capable of realizing the general class of coupled cavity transfer functions with a 50% reduction in size and weight compared to the two above-mentioned filters. The dual mode filter, shown in FIG. 1(c) is composed of a plurality of cascaded cavities each resonating in two independent orthogonal modes and the use of a reflective plate at one end of the filter enables both input and output ports to be taken from the same physical cavity at the opposite end of the filter. Intercavity coupling is provided by coupling screws 16 and intracavity coupling is provided by polarity discriminating irises between the physical cavities. The structure shown in FIG. 1(c) allows coupling between cavities 1 and n, 2 and (n-1), etc., so that the general class of filter transfer functions may be achieved.
Although these filters have proven quite advantageous, each has encountered a common disadvantage in that the location of the input/output ports has been fixed. Since the input and output ports of the two single mode waveguide filters must be taken from one end of the filters, the design flexibility of systems utilizing those filters has been somewhat limited.
Similarly, the input and output ports of the dual mode waveguide filter must be taken from the same physical cavity at one end of the cascaded cavity structure and, as in the case with the single mode filters, this requirement on the location of the input/output ports has substantially limited the physical design flexibility of systems utilizing the dual mode filter. Furthermore, since the input/output ports are taken from the same physical cavity, spurious out-of-band coupling occurs and must be carefully controlled. For most practical purposes, this requires the use of a coaxial waveguide transition which results in substantial and extremely undesirable weight and volume increases.
It would be particularly advantageous to be able to locate the input and output ports of the single and dual mode filters at other positions around the periphery of the filter structure in order to provide increased system design flexibility. It would also be advantages to provide physically separate input and output cavities in the dual mode filter to increase isolation and eliminate spurious out-of-band coupling.